Process and plant for the production of methanol

ABSTRACT

The present invention relates to a process for the production of methanol from a hydrocarbon feedstock comprising: contacting a vaporous mixture comprising the feedstock and steam in a steam reforming zone with a catalyst effective for catalysis of at least one reforming reaction; recovering from the reforming zone a synthesis gas mixture comprising carbon oxides, hydrogen and methane; supplying material of the synthesis gas mixture to a methanol synthesis zone charged with a methanol synthesis catalyst and maintained under methanol synthesis conditions; recovering from the methanol synthesis zone a product gas mixture comprising methanol and unreacted material of the synthesis gas mixture; supplying material of the product gas mixture to a methanol recovery zone maintained under methanol recovery conditions; recovering from the methanol recovery zone a crude methanol product stream and a vaporous steam comprising unreacted material of the synthesis gas mixture; separating material of the synthesis gas mixture into a first hydrogen-rich stream and a second hydrogen-depleted stream comprising carbon oxides and methane; supplying at least part of the first hydrogen-rich stream to the steam reforming zone as fuel; and recycling at least part of the second hydrogen-depleted stream to the steam reforming zone to form part of the mixture of the vaporous mixture comprising the feedstock and steam and to a plant constructed and arranged so as to be operable in accordance with the process.

This application is a 371 of PCT/GB97/03413 filed on Dec. 11, 1997.

FIELD OF THE INVENTION

This invention relates to a process and plant for the production ofmethanol.

BACKGROUND OF THE INVENTION

Methanol is synthesised in large volumes annually by conversion of acarbonaceous feedstock, usually a hydrocarbonaceous feedstock such asnatural gas, into a mixture of carbon oxides and hydrogen. Such amixture of gases is often referred to as synthesis gas.

The conversion of a hydrocarbon-containing feedstock, such as naturalgas, into synthesis gas can be effected by steam reforming.

In steam reforming a mixture of desulphurised hydrocarbon feedstock,such as natural gas, and steam is passed at high temperature, typicallyat a temperature of from about 600° C. to about 1000° C., and elevatedpressure, typically from about 10 bar up to about 50 bar, over asuitable reforming catalyst, such as a supported nickel catalyst. Onecommercially recommended catalyst which can be used for this purposeuses a mixture of calcium and aluminium oxides as support for thenickel. When natural gas is the feedstock, the principal reaction is:

CH₄+H₂O⇄CO+3H₂

The reaction products themselves are further subject to the reversible“water gas shift” reaction in which carbon dioxide and hydrogen areproduced from carbon monoxide and steam:

CO+H₂O⇄CO₂H₂

Conversion of the carbon oxides and hydrogen to methanol occursaccording to the following reactions:

CO+2H₂⇄CH₃OH

CO₂+3H₂⇄CH₃OH+H₂O

These reactions are conventionally carried out by contacting thesynthesis gas with a suitable methanol synthesis catalyst under anelevated synthesis gas pressure, typically in the range of from about 50bar up to about 100 bar, usually about 80 bar, and at an elevatedmethanol synthesis temperature, typically from about 210° C. to about270° C. or higher, e.g. up to about 300° C.

A conventional methanol synthesis plant can be considered to comprisefour distinct parts, namely:

1. a reforming plant, which produces a mixture of carbon oxides andhydrogen from a hydrocarbon feedstock;

2. a compression stage lifting the carbon oxides and hydrogen mixture toa higher pressure suitable for downstream methanol synthesis;

3. a methanol synthesis section, in which crude methanol is producedfrom the carbon oxides and hydrogen; and

4. a distillation section, in which the final refined methanol productis produced from the crude methanol.

Such a plant is described, for example, in WO-A-96/21634.

In order to achieve high yields of methanol, prior art processes havecommonly included a recycle loop around the methanol synthesis zone sothat unreacted materials leaving the methanol synthesis zone arerecycled to the methanol synthesis zone. Thus, U.S. Pat. No. 4,968,722relates to a process for the production of methanol by reacting carbonmonoxide and hydrogen in which the reactants are introduced into amethanol synthesis zone comprising one or more fixed catalyst beds. Theeffluent from the methanol synthesis zone is fed to an absorption zonewhere methanol is absorbed. Unreacted reactants are fed to a furthermethanol synthesis and recovery zone.

U.S. Pat. No. 5,472,986 discloses a methanol production process in whichhydrogen is recovered by use of a membrane from a purge gas taken fromthe methanol synthesis zone. The purged and separated hydrogen isrecycled to the methanol synthesis zone as a reactant for methanolsynthesis.

U.S. Pat. No. 4,181,675 relates to a methanol synthesis process in whichsynthesis gas is passed over a methanol synthesis catalyst in a methanolsynthesis zone and is then cooled to condense methanol. The remaininggas is recycled to the methanol synthesis zone. A purge stream from thisrecycle stream may be passed through a membrane to control any build upof inert gases in the recycle stream. Inert materials are separated fromcarbon oxide and hydrogen, the latter being supplied to the methanolsynthesis zone as reactants for methanol synthesis.

DE-A-3244302 discloses a process for the production of methanol in whichunreacted methanol synthesis gas is supplied to a three-way separationstage. In the separation stage, CO is separated and recycled to themethanol synthesis zone; CO₂ is separated and supplied to the reformingzone in order to replace part of the water vapour required forreforming; and a residual gas comprising hydrogen, nitogen and methaneis supplied to the reforming zone as fuel to heat the reformer tubes.

Various other methanol Production processes are known in the art, andreference may be made, for example, to U.S. Pat. No. 5,063,250, U.S.Pat. No. 4,529,738, U.S. Pat. No. 4,595,701, U.S. Pat. No. 5,063,250,U.S. Pat. No. 5,523,326, U.S. Pat. No. 3,186,145, U.S. Pat. No. 344,002,U.S. Pat. No. 3,598,527, U.S. Pat. No. 3,940,428, U.S. Pat. No.3,950,369 and U.S. Pat. No. 4,051,300.

A number of different types of reformer are known in the art. One suchtype is known as a “compact reformer” and is described in WO-A-94/29013,which discloses a compact endothermic reaction apparatus in which aplurality of metallic reaction tubes are close-packed inside a reformervessel. Fuel is burned inside the vessel, which comprises air and fueldistribution means to avoid excessive localised heating of the reactiontubes. In a compact reformer of this type heat is transferred from theflue gas vent and from the reformed gas vent of the reformer to incomingfeedstock, fuel and combustion air. Other types of reformer are not asefficient as the compact reformer in transferring heat internally inthis way. However, many other reformer designs are known and some aredescribed in EP-A-0033128, U.S. Pat. No. 3,531,263, U.S. Pat. No.3,215,502, U.S. Pat. No. 3,909,299, U.S. Pat. No. 4,098,588, U.S. Pat.No. 4,692,306, U.S. Pat. No. 4,861,348, U.S. Pat. No. 4,849,187, U.S.Pat. No. 4,909,808, U.S. Pat. No. 4,423,022, U.S. Pat. No. 5,106,590 andU.S. Pat. No. 5,264,008.

In a conventional plant, synthesis gas is compressed in passage from thereforming plant to the methanol synthesis zone. The synthesis gascompression stage is essentially present in order to provide therequired pressure of from 50 bar to 100 bar in the methanol synthesiszone. The synthesis gas compressor is an expensive item which has asignificant impact on the overall cost of the plant. Furthermore, thepresence in the plant of synthesis gas at such high pressuresnecessitates the use in the plant of thick walled stainless steel oralloyed steel high pressure pipework. This pipework is expensive to buy,to weld and to use as a construction material. It therefore represents asubstantial financial cost in the building of the plant.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a plant for methanolproduction which is cost-efficient to build and which avoids the use ofat least some of the expensive components hitherto favoured inconventional methanol plants. A further object of the invention is toprovide a process for the production of methanol which iscarbon-efficient, providing good yields of methanol and which does notrely essentially on the use of very high pressure in the methanolsynthesis zone. It is yet another object of the invention to provide amethanol production plant which is suitable for construction andoperation in remote or offshore locations.

According to the present invention, there is provided a plant for theproduction of methanol from a hydrocarbon feedstock material comprising:

a) a steam reforming zone, adapted to be maintained under steamreforming conditions and charged with a catalyst effective for catalysisof at least one steam reforming reaction, for steam reforming of avaporous mixture of the hydrocarbon feedstock and steam to form asynthesis gas mixture comprising carbon oxides, hydrogen and methane;

b) a methanol synthesis zone, adapted to be maintained under methanolsynthesis conditions and charged with a methanol synthesis catalyst, forconversion of material of the synthesis gas mixture to a product gasmixture comprising product methanol and unreacted material of thesynthesis gas mixture;

c) a methanol recovery zone, adapted to be maintained under methanolrecovery conditions, for recovery of a crude methanol product streamfrom the product gas mixture, and for recovery of a vaporous streamcomprising unreacted material of the synthesis gas mixture;

d) a separation zone for separation of material of the synthesis gasmixture into a first hydrogen-rich stream and a second hydrogen-depletedstream comprising carbon oxides and methane;

e) means for supplying at least part of the first hydrogen-rich streamto the steam reforming zone as fuel; and

f) means for recycling at least part of the second hydrogen-depletedstream to the reforming zone for admixture with the vaporous mixture ofhydrocarbon feedstock and steam.

The separation of the first hydrogen-rich stream from the secondhydrogen-depleted stream may occur upstream or downstream of themethanol synthesis zone. Thus, in one preferred embodiment of theinvention the separation zone is located downstream of the methanolsynthesis zone, means being provided for supplying the at least part ofthe second hydrogen-depleted stream from the separation zone to thereforming zone without passing through the methanol synthesis zone. Inan alternative embodiment of the invention, the separation zone islocated upstream of the methanol synthesis zone, means being providedfor supplying the at least part of the second hydrogen-depleted streamto the methanol synthesis zone and thereafter recovering an unreactedpart of the second hydrogen-depleted stream and supplying the unreactedpart to the reforming zone.

Usually, the carbon oxides referred to will comprise a mixture of CO andCO₂.

The invention further provides a process for the production of methanolfrom a hydrocarbon feedstock comprising:

a) contacting a vaporous mixture comprising the feedstock and steam in asteam reforming zone with a catalyst effective for catalysis of at leastone reforming reaction;

b) recovering from the reforming zone a synthesis gas mixture comprisingcarbon oxides, hydrogen and methane;

c) supplying material of the synthesis gas mixture to a methanolsynthesis zone charged with a methanol synthesis catalyst and maintainedunder methanol synthesis conditions;

d) recovering from the methanol synthesis zone a product gas mixturecomprising methanol and unreacted material of the synthesis gas mixture;

e) supplying material of the product gas mixture to a methanol recoveryzone maintained under methanol recovery conditions;

f) recovering from the methanol recovery zone a crude methanol productstream and a vaporous stream comprising unreacted material of thesynthesis gas mixture;

g) separating material of the synthesis gas mixture into a firsthydrogen-rich stream and a second hydrogen-depleted stream comprisingcarbon oxides and methane;

h) supplying at least part of the first hydrogen-rich stream to thesteam reforming zone as fuel; and

i) recycling at least part of the second hydrogen-depleted stream to thesteam reforming zone to form part of the mixture of step a)

The separation step may take place upstream or downstream of themethanol synthesis zone. Thus, it may be preferred that the separationstep g) takes place downstream of the methanol synthesis zone, the atleast part of the second hydrogen-depleted stream being supplied fromthe separation step g) to the reforming zone without passing through themethanol synthesis zone. Alternatively, it may be preferred that theseparation step g) takes place upstream of the methanol synthesis zone,the at least part of the second hydrogen-depleted stream being suppliedto the methanol synthesis zone, an unreacted part of the secondhydrogen-depleted stream being recovered thereafter and supplied to thereforming zone.

The process and plant of the invention have significant advantages overconventional plants and processes for the production of methanol, aswill now be described.

The process and plant of the invention operate such that unreactedcarbon oxides and methane recovered from the methanol synthesis zoneare, after separation from hydrogen, recycled as feedstock to thereforming zone. Hydrogen recovered from the separation zone is suppliedto the reforming zone as fuel. This arrangement differs from prior artarrangements in which unconverted synthesis gas, usually afterenrichment in either hydrogen or carbon oxides,is recycled to themethanol synthesis zone and has a number of significant advantages oversuch prior art processes. In the process and plant of the invention,there is provided a recycle circuit for unconverted carbon-containingcompounds, the reforming zone and the methanol synthesis zone beinginside the same recycle circuit. By “carbon-containing compounds” ismeant principally carbon oxides, methane, or mixtures thereof. By“carbon oxides” is meant principally carbon monoxide and carbon dioxide.

The recycle of unconverted carbon oxides and methane to the reformingzone means that, overall, the process of the invention is highly carbonefficient, with little or no carbon being lost from the process,regardless of the conversion yields obtained in either or both of thereforming zone and the methanol synthesis zone. Thus, the operator of aplant designed in accordance with the invention has the option tooperate the process of the invention at relatively low conversion yieldsper pass in one or both of the reforming zone and the methanol synthesiszone. This has potential cost-saving advantages. For example, themethanol synthesis zone may be operated at lower pressure and/or with asmaller catalyst volume than in conventional processes.

In the steam reforming zone of a plant according to the invention andoperated in accordance with the process of the invention, the degree ofconversion of the feedstock to synthesis gas may be maintained at a lowlevel, relative to conventional plants, because the hydrogen-depletedstream comprising unreacted carbon oxides and methane is recycled asfeedstock to the reforming zone in any event. The synthesis gas mixturerecovered from the steam reforming zone in the plant and process of theinvention comprises hydrogen, carbon oxides and methane. If the steamreforming zone is maintained under conditions such that the overallconversion of hydrocarbon feedstock to carbon oxides and hydrogen isrelatively low, methane will be present in the synthesis gas mixture inlarger quantities than if the conversion is high, in which case methanewill be present in relatively smaller quantities in the synthesis gasmixture. This is the case regardless of whether the hydrocarbonfeedstock is predominantly methane (as in natural gas) or whether thehydrocarbon feedstock is predominantly composed of some higherhydrocarbon. Higher hydrocarbons which are not steam reformed to carbonoxides and hydrogen are hydrocracked under the steam reformingconditions to methane. Thus, an ethane feedstock, a propane feedstock ora mixed butane/methane feedstock, for example, will reform to give amixture of carbon oxides, hydrogen and methane.

In conventional plants, it is desirable to ensure that reforming ofhydrocarbon to carbon oxides and hydrogen is as complete as possible.Thus, because low pressure favours the steam reforming reactions, it isdesirable in conventional plants to maintain the reforming zone under arelatively low pressure, for example about 20 bar. Whilst it iscertainly possible to operate the process and plant of the inventionsuch that a pressure of about 20 bar is used in the reforming zone, inpractice it is a desirable feature of the invention that higherreforming pressures, for example from about 25 bar to about 50 bar, forexample about 30 bar can be used. This has important advantagesdownstream of the reforming zone. In conventional processes, a make upgas compressor is used to compress the synthesis gas mixture enteringthe methanol synthesis loop. In addition, a recycle compressor isprovided within the loop to circulate unreacted synthesis gas therein.In the process of the invention, because the reforming zone is includedwithin a recycle circuit it is possible to provide a single compressorto drive the supply of the make-up gas to the methanol synthesis zoneand the recirculation of unreacted synthesis gas around the circuit.Moreover, the provision of a single circuit including the reformer meansthat the position of the compressor may be selected by the designer ofthe plant as desired. When only one compressor is used in this way, theplant of the invention may be significantly more compact than prior artplants. Thus, driving equipment and pipework associated with multiplecompression in the prior art is much reduced. This is significantbecause the plant of the invention may be built conveniently in remote,even offshore, locations. It has not hitherto been possible economicallyto construct a commercial methanol plant in an offshore location.

It is therefore an important feature of the present invention that theunreacted material of the synthesis gas mixture recovered from themethanol recovery zone, or the material of the synthesis gas mixturerecovered from the reforming zone as the case may be, compriseshydrogen, carbon oxides and methane and is separated into ahydrogen-rich stream, which is supplied as fuel to the steam reformingzone, and a hydrogen-depleted stream, comprising carbon oxides andmethane, which is recycled to the steam reforming zone for admixturewith the feedstock. The plant and process of the invention thereforeincludes the reforming zone, the methanol synthesis zone, the methanolrecovery zone and the separation zone inside one carbon oxide andmethane recycle circuit. This arrangement enables the plant and processof the invention to be operated with a single compression stage drivingthe flow of materials around the recycle circuit. The compression stagemay be provided at any convenient location inside the recycle circuit,the position of the compressor depending upon the balance betweencapital and operating costs of the plant. This contrasts withconventional processes, in which unconverted carbon oxides are recycledto the front end of the methanol synthesis zone and a recycle compressormust be provided to maintain the pressure or the recycle stream at thehigh pressures used in conventional methanol synthesis plants. Inconventional processes, it is not desirable to have a large quantity ofmethane present in this recycle stream and so a purge stream may betaken to control any build up of methane present in the synthesis gasmixture as a result of incomplete reaction in a reforming zone.

When the degree of conversion in the reforming zone is maintained at arelatively low level, this has little or no impact on the overallmethanol yield of a process in accordance with the invention becauseunconverted methane is recycled to the reforming zone in any event. Thisenables the use, in the process and plant of the invention, of arelatively low steam to carbon ratio and/or a relatively low outlettemperature in the reforming zone. Thus, in the process of the inventionthe steam to carbon ratio in the steam reforming zone is preferably lessthan about 3:1, even more preferably less than about 2.8:1, for exampleabout 2.5:1 or less. The outlet temperature of the reforming zone, bywhich is meant the temperature at the exit end of the reforming catalystinside the zone, may range from about 700° C. to about 1000° C., forexample about 850° C. The use of a lower reforming temperature, comparedto conventional plants, allows the operator of a plant and processaccording to the invention to use a relatively high reforming pressure,for example a reforming pressure of more than about: 20 bar, for exampleabout 30 bar or about 40 bar or more. In particular, the use of a“compact reformer”, as described in WO-A-94/29013, operated atrelatively low temperatures and relatively high pressures allows a plantaccording to the invention to be significantly more compact thanconventional plants. This is significant because a plant according tothe invention may conveniently be built in remote, even offshore,locations. It has not hitherto been possible economically to construct acommercial methanol plant in an offshore location.

The process and plant of the invention have great flexibility and may bedesigned such that in the methanol synthesis zone the conversion yieldper pass of carbon oxides to methanol is from about 40% to about 95% orhigher, preferably from 70% to 90% for example about 80%.

The process and plant of the present invention preferably utilisepressures of from about 20 bar to about 50 bar, e.g. from about 35 barto about 45 bar, e.g. about 40 bar in the methanol synthesis zone.

The use of relatively low pressures in the methanol synthesis zone hasthe further advantage that the cost of building a plant in accordancewith the invention is significantly reduced, relative to conventionalplants, by avoiding the need to use thick-walled, high pressurepipework.

In conventional plants, a synthesis gas compressor is required to drivethe synthesis gas into the methanol synthesis zone at a pressure ofabout 80 bar. Typically, the motive force for gas compression isprovided by high pressure steam generated within the plant by a steamturbine. The plant and process of the invention may be operated at muchlower pressures, as has been explained above. The process of theinvention can use a smaller compressor than has been used in prior artprocesses. The pressure in the methanol synthesis zone of the plant ofthe invention may be provided by a single compression stage which may belocated at any suitable position inside the recycle circuit.

The possibility to operate the plant of the invention with only onerelatively small compressor has ramifications beyond cost. The absenceof any associated steam turbine, steam generation and transfer system,significantly reduces the size of a plant according to the invention, inrelation to conventional plants. This reduction in size allows the plantof the invention to be constructed economically in remote or offshorelocations.

In conventional plants, the fuel used in the steam reforming zone isgenerally a hydrocarbon feedstock material which may contain sulphurousimpurities such as hydrogen sulphide. In the plant and process of thepresent invention, the separated hydrogen-rich stream is supplied asfuel to the reforming zone. The flue gas from the reforming zone of aplant according to the invention is therefore substantially sulphur freeand can, if desired, be cooled below its dew point for immediatedisposal, without the need for further treatment to remove sulphurousacids, as may be required in conventional plants.

If desired, a purge stream may be taken from the carbon oxide andmethane containing recycle stream. The purge stream may be supplied asfuel to the reforming zone. Usually, a purge stream will be taken, therate of purge being selected to control any accumulation in the recyclecircuit of chemically inert materials, such as nitrogen, argon andhelium, that may be present in the feedstock material.

In a preferred plant and process of the invention, the separation zonecomprises a membrane separator which may be of any suitable design. Anumber of membrane separators suitable for use in the process and plantof the present invention are described in U.S. Pat. No. 4,181,675,referred to hereinabove.

It is further preferred that the methanol synthesis zone comprise anumber of methanol synthesis reactors connected in series. A methanolrecovery zone may be provided between each successive methanol synthesisreactor and after the last methanol synthesis reactor in the series. Avaporous carbon oxide and hydrogen-containing stream from each methanolrecovery zone, other than the last in the series, is supplied to a nextsuccessive methanol synthesis reactor in the series. The methanolsynthesis reactions are equilibrium limited and this arrangement has theadvantage that methanol is removed from the reaction mixture betweeneach methanol synthesis reactor, thereby favouring the methanol formingreactions in the next successive methanol synthesis reactor.

Methanol recovery may be achieved by any suitable method, such aschilling or solvent washing. If solvent washing is chosen, suitablesolvents include ethylene glycol, tetraethyleneglycol dimethyl ether,water and the like.

Conveniently, the or each crude methanol product stream is supplied to arefining zone for recovery of a refined methanol product stream. Therefining zone may be remotely located from the plant. Thus, if the plantis constructed in an offshore location, a crude methanol productcontaining about 6% water may be recovered from the methanol recoveryzone and shipped ashore for subsequent refining.

Desirably, a single gas compressor is provided to drive the feedstock,synthesis gas and vaporous carbon oxide and hydrogen-containing streams.The plant and process of the invention may be operated using a singlestage compressor when the methanol synthesis pressure is maintained ator beneath about 50 bar. If methanol synthesis pressures of over about50 bar are required, it may become desirable to employ a secondcompressor. The use of a single compressor has beneficial effects on thecost of building a plant in accordance with the invention and also onthe space occupied by such a plant. The use of a single compressor incombination with a compact reformer, of the type mentioned above,enables a plant according to the invention to be economicallyconstructed and operated at an offshore location. The provision ofoffshore methanol synthesis facilities is an important aspect of theinvention and represents a significant improvement on conventionalreformer based methanol synthesis technology, which cannot currently beprovided offshore on a cost-effective basis.

The methanol synthesis zone is preferably maintained at a temperature offrom about 210° C. to about 300° C., e.g. about 230° C. to about 270°C., e.g. about 240° C.

In a preferred process according to the invention, in which thereforming zone is a compact reforming zone of the type hereinbeforedescribed, combustion air supplied to the reforming zone is saturated orpartially saturated with water vapour before being supplied to thereforming zone. This has the advantage of modifying the combustioncharacteristics within the reforming zone, giving a more even heating ofreforming elements within the zone and a reduction in emissions ofnitrogen oxides, and carbon dioxide in the flue gas, relative toconventional plants.

In a preferred plant according to the invention the reforming zone is acompact reforming zone of the type hereinbefore described. However, thesteam reforming zone used in the process and plant of the invention maybe of any suitable design.

A preferred feedstock for use in the process of the invention is naturalgas.

An advantageous feature of the plant and process of the invention isthat the flue gas from the steam-reforming zone contains significantlylower quantities of carbon oxides and sulphur-containing compounds thana conventional plant of equivalent methanol production capacity.

In order that the invention may be clearly understood and readilycarried into effect, a number of methanol synthesis plants constructedand arranged in accordance with the invention and designed to operate apreferred process in accordance with the invention will now bedescribed, by way of example only, with reference to the accompanyingdiagrammatic drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified flow diagram of diagram of a first methanolsynthesis plant according to the invention;

FIGS. 2a, 2 b and 2 c combine to show a more detailed flow diagram of asecond methanol synthesis plant according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

It will be understood by those skilled in the art that the drawings arediagrammatic and that further items of equipment such as reflux drums,pumps, vacuum pumps, temperature sensors, pressure sensors, pressurerelief valves, control valves, Flow controllers, level controllers,holding tanks, storage tanks, and the like may be required in acommercial plant. The provision of such ancillary items of equipmentforms no part of the present invention and is in accordance withconventional chemical engineering practice.

Referring to FIG. 1, a stream of natural gas is supplied in line 101and, after passing through natural gas compressor 102, passes on in line103 at a pressure of around 40 bar. Feedstock compressor 102 is furthersupplied with a recycle stream of carbon oxides and methane from line104, as will be explained later.

The compressed feedstock and recycle stream in line 103 is supplied to afeed pretreatment zone 105. In feed pretreatment zone 105, the detailsof which are not shown in FIG. 1, the compressed stream is heated toaround 380° C. before passing on to a desulphurisation reactor. Thenatural gas feedstock contains a minor amount or sulphur as hydrogensulphide which is a poison to downstream catalysts. Sulphur is removedin passage through the desulphurisation reactor which contains a chargeof desulphurisation materials, such as nickel molybdate and zinc oxide.

The desulphurised gas is cooled by passage through an interchanger andflows into the bottom of a saturator column in which the gas flowscountercurrent to hot water supplied to the top of the saturator column.

In passage through the saturator column the gas mixture is saturatedwith water vapour. The water vapour-saturated gas mixture exits thesaturator at about 200° C. and contains approximately 90% of the steamrequired for subsequent reforming. The gas/steam mixture is mixed withfurther steam supplied from a gas turbine and passes on through a mixedfeed heater which is mounted in the flue gas duct of reformer 106. Inpassage through the mixed feed heater the temperature of the gas/steammixture is raised to about 400° C. The resulting hot gas is fed in line107 to reformer 106.

The detail of reformer 106 is not shown in FIG. 1. The reformer ispreferably of the compact type hereinbefore described. Hot gas from line107 is fed into the reaction tubes of compact reformer 106 which arepacked with a suitable steam reforming catalyst, for example a supportednickel catalyst. The heat required to drive the endothermic reformingreactions is supplied by burning a hydrogen-rich fuel inside compactreformer 106, thus transferring heat to the reaction tubes by radiationand convection.

Reformer 106 is fed with hot combustion air from line 108, which ispre-heated in a combustion air pre-heater (not shown) heated by reformedgas inside compact reformer 106 and pre-compressed in combustion aircompressor 109 after being supplied to the plant in line 110. Hydrogento fuel reformer 106 is supplied in line 111 from a downstreamseparation step, as will be explained later. Hydrogen is combustedinside reformer 106, thus supplying radiant and convective heat to thereformer reaction tubes. Flue gas is vented from reformer 106 in line112.

In compact reformer 106 the feed mixture of natural gas, steam andrecycled carbon oxides is reformed to a mixture of carbon monoxide,carbon dioxide, hydrogen and methane, a mixture commonly known assynthesis gas.

In the presence of the nickel catalyst at elevated temperatures, steamreacts with vaporous hydrocarbons at elevated temperatures and pressuresto give a synthesis gas consisting of carbon dioxide, carbon monoxide,and hydrogen, together with methane. The concentration of eachconstituent in the synthesis gas depends on the ratio of steam tohydrocarbon passing over the catalyst, and on the temperature andpressure at which the gases leave the catalyst. The reactions takingplace are complex but the end product is determined by two reactions,i.e.

(i) the water gas shift equilibrium reaction:

CO+H₂O⇄CO₂+H₂+Heat

(ii) the steam-methane equilibrium reaction

Heat+CH₄+H₂O⇄CO+H₂

Overall the reactions are endothermic. A large excess of steam and ahigh temperature are required to move the equilibrium to the right andto reduce the residual methane content of the synthesis gas.

The synthesis gas leaves compact reformer 106 in line 113 at about 450°C. and about 30 bar. In operation sufficient carbon oxides and/ormethane are preferably introduced through line 104 to provide astoichiometric synthesis gas in line 113; hence the rate of carbon oxideand/or methane recycle may be controlled so that, on a molar basis, thehydrogen content is equal to twice the carbon monoxide content plusthree times the carbon dioxide content.

The hot synthesis gas is cooled and passes by way of line 113 tomethanol converter 114.

Typical methanol synthesis conditions in accordance with the inventioninclude use of a pressure in the region of 30 bar and an outlettemperature of from about 210° C. to about 240° C. using a copper/zinccatalyst, for example the catalysts sold as ICI 51-7, Haldor TopsoeMK-101 or Súd-Chemie C79-5GL.

The methanol synthesis equilibria are as follows:

CO+2H₂⇄CH₃OH

CO₂+3H₂⇄CH₃OH+H₂O

Typically, the gas in line 113 contains about 10 to about 20 vol %carbon oxides, the balance being hydrogen, methane and nitrogen.Nitrogen can be present as an impurity in the natural gas feedstock.

A product mixture is recovered in line 115 and passed to a methanol washcolumn 116, from which a crude methanol product is recovered in line117. Unreacted synthesis gas from wash column 116 is supplied in line119 to a separation zone 120.

Separation zone 120 can operate using any convenient known technique,for example pressure swing absorption, membrane technology,liquefaction, or a combination of two or more thereof. The use ofmembrane technology is preferred, often being the most economical.

A hydrogen-rich recycle stream is recovered in line 121 and supplied inline 111 as fuel to compact reformer 106. A carbon oxide and/ormethane-rich stream is recovered in line 122 and supplied to line 104 asa recycle stream for admixture with the feedstock. A purge may be takenin line 123 to control any build up of inert materials.

Crude methanol product in line 117 is supplied to a refining zone 124,from which is recovered a refined methanol product in line 125.

Referring now to FIG. 2a, natural gas from battery limits is supplied tothe plant in line 201 and enters natural gas knockout drum 202 beforepassing on in line 203. A portion of the gas in line 203 is taken inline 204 to power gas turbine 205.

Hot gas from gas turbine 205 passes along line 206 into heat recoveryduct 207. Flue gas is vented to the atmosphere in line 208. Steam iswithdrawn in line 209 and separated into two streams in line 210 andline 211. Steam in line 210 is further separated into two streams inline 212 and line 213. Steam in line 213 is supplied to the steamreforming process, as will be described later. Steam in line 212 issupplied to a methanol refining process, as will be described later.Steam in line 211 passes into deaerator 214, which is vented in line215. Deaerated water is withdrawn in line 216 and passed via boilerwater pump 217 into line 218. Water in line 218 passes on in line 219and is fed to heat recovery duct 207. A make-up water stream is taken inline 220 and fed to a converter steam drum (not shown).

The remaining gas in line 203 passes on in line 221 and is compressed toaround 25 bar in natural gas compressor 222. Compressed gas passes on inline 223 and combines in line 224 with a recycle stream from line 225.The combined stream in line 224 is cooled through interchanger 226 whichis supplied with cooling water in line 227. The cooled stream passes onin line 228 and into knock out pot 229, where any condensate from thecooled stream is removed. The mixed gas stream then passes on in line230 and is compressed to around 38 bar in recycle compressor 231.

The compressed gas stream passes on in line 232, is heated throughinterchanger 233, passes on in line 234 and is further heated throughinterchanger 235 which is mounted in the flue gas stream from reformer236. Hot gas, now at a temperature of about 380° C., passes on in line237 and into desulphurisation vessel 238 which contains a charge 239 ofa suitable sulphurisation catalyst, such as nickel molybdate or cobaltmolybdate. In the plant of FIG. 2a, zinc oxide is used as catalyst.

Gas from desulphurisation vessel 238 flows on in line 241 todesulphurisation vessel 240, which contains a charge 242 of a zinc oxidedesulphurisation catalyst. The desulphurised gas stream, now containingless than about 0.1 parts per million of sulphur, flows on in line 243through interchanger 233, where it is cooled, and passes via line 244into the bottom of feed saturator 245.

Feed saturator 245 is supplied with hot water in line 246. Fresh wateris supplied to the plant in line 247 and is pumped by pump 248 intolines 249, 250, 251 and 252, through pump 253 and into line 254. Waterin line 254 is heated through interchanger 255 and is supplied in line256 to interchanger 257. The heated water or steam passes on in line 258to a further interchanger 259 and then into line 246.

In feed saturator 245 the mixed gas stream flows upwards and the hotwater stream flows downwards. The gas leaves saturator 245 in line 260containing around 90% of the steam required for downstream reformingreactions. The remaining 10% of steam is supplied in line 213 so that agas stream containing 100% of the steam required for steam reformingpasses on in line 261.

Water from the bottom of saturator 245 is recycled through lines 262 and263 to combine in line 215 with Fresh water from line 250. A smallblowdown taken from stream 262 passes on in line 264 for disposal. Awarm water stream proceeds in lines 251 and 252 and is pumped by pump253 into line 254, through interchanger 255, line 256, interchanger 257,line 258, interchanger 259 and into line 246 for supply to the top ofsaturator 245. The remainder of the blowdown stream from line 260 passeson in line 264 for disposal.

The gas stream in line 261 is heated in passage through interchanger 265and passes on in line 266 to reformer 236. Interchangers 235, 265 and257 are mounted in the flue gas duct of reformer 236. Interchangers 259and 255 are mounted in the reformed gas duct of reformer 236. Reformer236 comprises, in the plant shown in FIG. 2a, a number of compactreformer tubes arranged in parallel with each other. A reformingcatalyst (not shown), such as a supported nickel catalyst, is providedwithin the reformer tubes. The feedstock and steam mixture from line266, now at a temperature of about 400° C., passes into reformer 236 andflows therethrough from top to bottom.

The heat to drive the endothermic reforming reactions is supplied byburning a hydrogen-rich fuel inside reformer 236. Hydrogen fuel issupplied to reformer 236 in line 267. The fuel is recycled from adownstream separation process, as will be described later.

Combustion air for compact reformer 236 is supplied to the plant in line268 and passes by means of air compressor 269 into line 270 and theninto an air saturator column 271. The purpose of saturating thecombustion air is to control the heat recovery inside compact reformer236, to allow greater recovery of energy within the plant. Hot water issupplied to air saturator column 271 in line 272 after being recycledfrom a downstream refining step, as will be explained later. Water fromthe bottom of air saturator column 271 in line 273 is cooled in passagethrough heat exchanger 274 supplied with cooling water in line 275. Thecooled water stream passes on in line 276 and is combined in line 277with fresh water from line 278 before being pumped by pump 279 into line280 for ultimate use in a downstream methanol recovery process, as willbe described later. A saturated combustion air stream emerging from thetop of air saturator column 271 is supplied to reformer 236 in line 281.

Although not shown in the plant of FIG. 2a, 2 b and 2 c, it is alsopossible to saturate the reformer fuel in line 267. It may be especiallypreferred to saturate the reformer fuel when the plant of the inventionuses a compact reformer, of the type hereinbefore described.

The use of compact reformer 236 means that much of the heat generatedwithin the reformer is recovered internally to reduce the overall fuelrequirements of the plant. Also, reformed gas and flue gas from reformer236 is used (in interchangers 255, 257 and 259) to heat the circulationwater for the feed saturator 245. The water is heated first by reformedgas in saturator water heater 255, then by flue gas in saturator waterheater 257 and finally by hot reformed gas in saturator water heater259. The arrangement of heat exchangers can be modified to suitalternate reformer designs. The arrangement depicted in FIG. 2 takesadvantage of the compact reformer to provide a heat recovery system withno “heat recycle” from the synthesis section to the reforming section.This makes plant start-up both easier and quicker than in conventionalmethanol plants.

A synthesis gas mixture, comprising carbon oxides, hydrogen and methane,is recovered from reformer 236 in line 282 and is cooled throughinterchanger 259, line 283 and interchanger 255 before passing on inline 284. The reformed gas stream exiting saturator water heater 255 isused to provide from about 35% to about 40% of the reboil heat for adownstream distillation column, as will be described later.

A flue gas stream exits reformer 236 in line 283 a and exchanges heatwith various streams in passage through interchanger 235, line 284 a,interchanger 265, line 285 and interchanger 257 before passing into line286. The flue gas stream leaves the plant via stack 287.

Referring now to FIG. 2b, synthesis gas in line 284 is further cooled ininterchanger 370, by means of which reboil heat is supplied todistillation column 289. Cooled synthesis gas is passed by line 290 toknock out pot 291. Condensate from knock out pot 291 is supplied vialine 292, pump 293 and line 294 to, and referring back now to FIG. 2a,line 295, line 296 and is then combined in line 252 with water from line251. The combined stream in line 252 is eventually supplied to feedsaturator column 245, as hereinbefore described.

Referring back to FIG. 2b, a synthesis gas mixture is recovered from thetop of knock out pot 291 in line 297 and passes through interchanger 298where it is cooled, supplying heat to a crude methanol stream suppliedto distillation column 289, as will be described later. The cooledsynthesis gas stream from interchanger 298 passes on in line 299. Thestream in line 299 passes through interchanger 300, where it is used topre-heat demineralised water for supply to the process as steam, as willbe now be described.

Interchanger 300 is supplied in line 301 with demineralised watersupplied to the plant via, and referring briefly back to FIG. 2a, line302 and pump 303. Referring back to FIG. 2b, heated demineralised waterpasses on in line 304 and into, referring briefly back to FIG. 2a,deaerator 214.

Referring back to FIG. 2b, further cooled synthesis gas frominterchanger 300 passes on in line 305 to gas cooler 306, line 307,interchanger 308 supplied with cooling water in line 309, into line 310and is supplied to a second knockout pot 311. Condensate from knock outpot 311 is recovered in line 312 and is supplied, via pump 313 and line314, to, and referring back to FIG. 2a, line 296 and is combined in line252 with make-up water from line 250 and 251.

Referring back to FIG. 2b, synthesis gas emerging from the top of knockout pot 311 is supplied in line 315 to, and referring now to FIG. 2c,interchanger 316., through which it is pre-heated to a methanolsynthesis temperature of about 210° C. before passing on in line 317 tomethanol synthesis reactor 318 containing a charge 319 of a methanolsynthesis catalyst, such as a copper/zinc catalyst, e.g. the catalystsold under the designation Haldor Toopsoe MK-101. In the illustratedmethanol converter 318, the exothermic heat of reaction is removed byraising steam in tubes mounted in the hot catalyst bed.

A circulation loop around methanol converter 318 is formed by line 320,converter steam drum 321 and line 322. Make-up water to the convertersteam drum 321 is supplied from line 220 (FIG. 2a) via a connecting line(not shown). Product steam from converter steam drum 321 in line 323 issupplied to line 324, where it combines with steam from line 212, and isultimately supplied as a reboiler heat to distillation column 289, aswill be explained later.

A product gas mixture comprising methanol, carbon oxides, methane andhydrogen is recovered from methanol converter 318 in line 325. Thestream in line 325 is cooled through interchanger 316 and passes on inline 326 to methanol wash column 327 which is supplied with wash waterin line 328. If desired, an additional cooler (not shown) can beincorporated in line 326. Referring briefly to FIG. 2a, line 328 issupplied with wash water from line 280.

Crude methanol product is recovered from methanol wash column 327 inline 329 and is passed through a filter 330 into line 331 and on intoline 332 for ultimate supply to a downstream refining step, as will bedescribed later.

Synthesis gas mixture emerging from the top of methanol wash column 327is passed in line 333 to a second methanol synthesis loop identical tothe loop just described. A third and a fourth loop are also provided.

On exiting the fourth methanol wash column 334, unreacted synthesis gasmixture is supplied in line 335 to interchanger 336 and on into membraneseparator 337. Hydrogen passes through membrane 338 and exits separator337 in line 339, from where it passes on in line 267 to, and referringbriefly to FIG. 2a, reformer 236. Carbon oxides and unreacted feedstockdo not pass through membrane 338 and exit separator 337 in line 340.

A purge stream may be taken from line 340 in line 341 to control anybuild up of inert materials in the recycle stream. Purge line 341 iscontrolled by valve 342.

After the purge, if any, the recycle stream in line 340 passes on inline 225 and, referring back to FIG. 2a, is combined in line 224 withmake-up natural gas from line 223.

Referring back to FIG. 2c, crude methanol product in line 332 issupplied, and referring now to FIG. 2b, via interchanger 298 to line343. Crude methanol product in line 343 is supplied to the middle of amethanol refining column 289.

Refined methanol product is recovered from near the top of column 289 inline 344. The refined stream in line 344 is cooled through interchanger345, supplied in line 346 by cooling water, and passes into line 347 andinto methanol shift tank 348. Product methanol is recovered from shifttank 348 via line 349, pump 350 and line 351.

Vaporous material exits the top of column 289 in line 352 and is passedthrough condenser 353. Product from condenser 353 is recovered in line354, which is vented in line 355. Unvented material flows on in line 35Gto column reflux drum 357, before being recycled in line 358, via pump359 and line 360, to the top of column 289. The vented stream in line355 is cooled through heat exchanger 361, cooled by cooling water inline 362, and passes on in line 363 and line 354 to column reflux drum357. Gas in line 363 could be recovered by suitable compression but hereis vented in line 365 to the atmosphere.

A bottoms product is recovered from column 289 in lines 366, 367 and368. The stream in line 366 is supplied via pump 369 to line 272 and,referring briefly back to FIG. 2a, to combustion air saturation column271.

Referring back to FIG. 2b, bottoms product in line 367 is recycled tothe bottom of column 289 via interchanger 370 and line 371. Bottomsproduct in line 368 is recycled to the bottom of column 289 viainterchanger 372 and line 373.

What is claimed is:
 1. A process for the production of methanol from ahydrocarbon feedstock comprising: a) contacting a vaporous mixturecomprising the hydrocarbon feedstock and steam in a steam reforming zonewith a catalyst effective for catalysis of at least one reformingreaction; b) recovering from the reforming zone a synthesis gas mixturecomprising carbon oxides, hydrogen and methane; c) supplying material ofthe synthesis gas mixture to a methanol synthesis zone charged with amethanol synthesis catalyst and maintained under methanol synthesisconditions; d) recovering from the methanol synthesis zone a product gasmixture comprising methanol and unreacted material of the synthesis gasmixture; e) supplying material of the product gas mixture to a methanolrecovery zone maintained under methanol recovery conditions; f)recovering from the methanol recovery zone a crude methanol productstream and a vaporous stream comprising unreacted material of thesynthesis gas mixture; g) separating material of the synthesis gasmixture into a first hydrogen-rich stream and a second hydrogen-depletedstream comprising carbon oxides and methane; h) supplying at least partof the first hydrogen-rich stream to the steam reforming zone as fuel;and i) recycling at least part of the second hydrogen-depleted streamcomprising carbon oxides and methane to the steam reforming zone to formpart of the vaporous mixture of step a).
 2. A process according to claim1, wherein the separation step g) takes place downstream of the methanolsynthesis zone, the at least part of the second hydrogen-depleted streambeing supplied from the separation step g) to the reforming zone withoutpassing through the methanol synthesis zone.
 3. A process according toclaim 1, wherein the separation step g) takes place upstream of themethanol synthesis zone, the at least part of the secondhydrogen-depleted stream being supplied to the methanol synthesis zone,an unreacted part of the second hydrogen-depleted stream being recoveredthereafter and supplied to the reforming zone.
 4. A process according toany one of claim 1, wherein the methanol synthesis zone is maintainedunder a pressure of from about 20 bar to about 50 bar.
 5. A processaccording to claim 4, wherein the methanol synthesis zone is maintainedunder a pressure of from about 25 bar to about 40 bar.
 6. A processaccording to claim 5, wherein the methanol synthesis zone is maintainedunder a pressure of about 30 bar.
 7. A process according to claim 1,wherein the separation of the first hydrogen-rich stream from the secondhydrogen-depleted stream is achieved by means of a membrane separator.8. A process according to claim 1, wherein the methanol synthesis zonecomprises a plurality of methanol synthesis reactors connected inseries, each successive pair of methanol synthesis reactors beingseparated by a methanol recovery zone, wherein the product gas mixturefrom each methanol synthesis reactor in the series is supplied to acorresponding methanol recovery zone and unreacted material of thesynthesis gas mixture recovered from the methanol recovery zone issupplied to the next successive methanol synthesis reactor in theseries.
 9. A process according to claim 1, wherein the crude methanolproduct stream is supplied to a refining zone for recovery of a refinedmethanol product stream.
 10. A process according to claims 1, wherein asingle gas compressor is provided to drive the feedstock, synthesis gasand recycle streams.
 11. A process according to claims 1, wherein themethanol synthesis zone is maintained at a temperature of from about200° C. to about 300° C.
 12. A process according to claims 1, whereinthe feedstock comprises natural gas.
 13. A plant for the production ofmethanol from a hydrocarbon feedstock material comprising: a) a steamreforming zone, adapted to be maintained under steam reformingconditions and charged with a catalyst effective for catalysis of atleast one steam reforming reaction, for steam reforming of a vaporousmixture of the hydrocarbon feedstock and steam to form a synthesis gasmixture comprising carbon oxides, hydrogen and methane; b) a methanolsynthesis zone, adapted to be maintained under methanol synthesisconditions and charged with a methanol synthesis catalyst, forconversion of material of the synthesis gas mixture to a product gasmixture comprising product methanol and unreacted material of thesynthesis gas mixture; c) a methanol recovery zone, adapted to bemaintained under methanol recovery conditions, for recovery of a crudemethanol product stream from the product gas mixture, and for recoveryof a vaporous stream comprising unreacted material of the synthesis gasmixture; d) a separation zone for separation of material of thesynthesis gas mixture into a first hydrogen-rich stream and a secondhydrogen-depleted stream comprising carbon oxides and methane; e) meansfor supplying at least part of the first hydrogen-rich stream to thesteam reforming zone as fuel; and f) means for recycling at least partof the second hydrogen-depleted stream comprising carbon oxides andmethane to the steam reforming zone for admixture with the vaporousmixture of hydrocarbon feedstock and steam.
 14. A plant according toclaim 13, wherein the separation zone is located downstream of themethanol synthesis zone, means being provided for supplying the at leastpart of the second hydrogen-depleted stream from the separation zone tothe reforming zone without passing through the methanol synthesis zone.15. A plant according to claim 13, wherein the separation zone islocated upstream of the methanol synthesis zone, means being providedfor supplying the at least part of the second hydrogen-depleted streamto the methanol synthesis zone and thereafter recovering an unreactedpart of the second hydrogen-depleted stream and supplying the unreactedpart to the reforming zone.
 16. A plant according to claim 13, whereinthe separation zone comprises a membrane separator.
 17. A plantaccording to claim 13, wherein a plurality of methanol synthesis zonesare provided in series with a plurality of methanol recovery zones, therecycle stream from each methanol recovery zone, other than the last inthe series, being supplied to a next successive methanol synthesis zonein the series.
 18. A plant according to claim 13, wherein there isfurther provided a refining zone, maintained under refining conditions,having an inlet for supply of the crude methanol product stream and anoutlet for recovery of a refined methanol product stream.
 19. A plantaccording to claim 13, wherein a single gas compressor is provided todrive the feedstock, synthesis gas and recycle streams.